Breathing training (BT) has long been proposed as an adjunctive treatment for asthma, but little controlled evidence is available on its efficacy. Recently, a hypoventilation training has been recommended that claims improvements in asthma control by raising basal pCO2 levels. Physiological research supports the rationale of the technique, and a number of controlled trials have demonstrated limited benefits of the training. However, because none of these studies has provided evidence for increases in basal pCO2 levels across training, the central assumption of the hypoventilation training idea remains untested. Based on this rationale and the evidence from respiratory physiology, we recently developed and pilot-tested a home-based pCO2 biofeedback assisted BT with a handheld capnometry device for asthma patients. The primary goal of this training was to teach patients how to increase their basal pCO2 levels. The results showed that this procedure was successful in relieving symptoms, improving control of asthma, and reducing diurnal variability of lung function, an objective indicator of asthma severity. We are proposing a 4-year study to systematically assess the efficacy of hypoventilation-based BT as an adjunct treatment for asthma. The study is a randomized, controlled trial comparing the short- and long-term benefits of 5 sessions of hypoventilation training compared to a control intervention of breathing regularity and awareness training. We will randomize a total of 120 asthma patients. The outcome will be evaluated by a multi-level standard measures test battery of asthma control including measures of pathophysiology (peak flow variability, airway inflammation by exhaled nitric oxide, airway hyperreactivity to methacholine, basal lung function), self- reported asthma control and symptoms, and self-medication behavior (bronchodilator use). Effects on, asthma management (corticoid inhaler adherence, perceived control and efficacy of management, perceived triggers), general well-being (perceived health, stress, anxiety, depression), and cardio-respiratory function in daily life (multi-system ambulatory respiratory inductance plethysmography, capnometry, spiromtery) will also be assessed. We expect substantial benefits of the capnometry-assisted breathing training on asthma control, We will also explore the role of improvements in asthma management and general well- being (perceived health, stress, anxious and depressed mood) through training as a mediators of change in asthma control, as well as the potential of exhaled nitric oxide to predict clinically significant change in other asthma control measures throughout therapy. This study will explore the benefits of a novel breathing training for asthma patients added to their regular medication treatment. The trainings aims at reducing hyperventilation, which is known to make asthma worse, but which has not been targeted successfully by prior breathing training methods. We expect our training to improve asthma control, reduce symptoms and medication needs, as well as improve patients'well-being. Public Health Relevance Paragraph: Research suggests that asthma patients may often hyperventilate, which makes their symptoms worse. In this project we will test a breathing training that reduces hyperventilation with the help of a hand-held device that helps patients monitor their breathing. Patients will participate in a four-week intervention with five training sessions and breathing home exercises, which is expected to improve their asthma control and well-being.